Gas metal arc welding (GMAW) involves applying electrical current to a contact tip in a welding torch through which a consumable electrode or welding wire is passed as it moves toward a workpiece. Electrical current connected to the welding wire by the contact tip creates the electric arc for the welding process in accordance with standard welding technology. A power source, normally a high speed switching inverter, creates the welding current and is normally regulated on the basis of arc voltage to maintain a stable arc length in a constant voltage welding or pulse welding. The electrode or welding wire advancing through the contact tip toward the workpiece is electrically connected to the workpiece by the electric arc having a length that should have a given length for a specific process. The spacing between the contact tip and the workpiece is the welding parameter known as the contact tip to work distance (CTWD). This parameter is distinguished from the electrode stickout (ESO) which is the length of the advancing welding wire measured from the contact tip to the arc and includes a summatio of ESO and arc length. In gas metal arc welding it is desirable to control CTWD to maintain a stable are length. CTWD varies due to process disturbances, such as weldment dimensional tolerances experienced during production, tooling/fixture positioning accuracy and part distortion during welding. To compensate for CTWD variations and, thus, maintain arc stability, it is normal procedure in GMAW processing to adjust the arc current to change the heat input and, thus, vary the burn-off rate of the wire. Changing arc current, while maintaining voltage constant, leads to substantial changes in the heat input per unit length of the weld bead and substantially affects the cooling rate of weld pool solidification. Using variations in the heat input by changing the current to adjust for variations in CTWD or arc length, has been found to cause corresponding changes in the weld surface profile, penetration profile, base metal dissolution, metallurgical properties and mechanical properties in the weld bead. Consequently, changing the current in a constant voltage welding process to compensate for changes in CTWD presents substantial process variations exhibited primarily in the heat affected zone. Further, distortion level of the weldment and quality of the weld structure is more difficult to maintain when using arc current to compensate for changes in CTWD and, thus, in changes of arc length. To avoid the necessity of increasing or decreasing arc current based upon variations in CTWD, especially on robotic applications, the arc current is measured and used to mechanically adjust the actual spacing between the contact tip and the workpiece. This procedure is often employed in seam tracking of robotic welding equipment wherein the torch weaves back and forth as it moves along a seam. The CTWD at each end of the weave is compared to determine if the torch is centered in the joint. Variations cause a change in the path of the torch to track the seam.
Because of the mechanical inertia and the dependency on a feedback loop process when adjusting CTWD as well as heat differences, a method using the arc current for arc stability is not used, especially in a high speed welding application. Another arrangement for physically adjusting the spacing of the tip with respect to the workpiece is a vision system that actually scans the position of the tip with respect to the workpiece during the welding operation. Such vision systems are not employed due to high cost and maintenance in hostile environments. In summary, the most accepted way of maintaining arc stability is to adjust the welding or arc current to compensate for variations in CTWD. This presents the problems heretofore explained in detail.